1. Field of the invention:
The present invention relates to a method for the formation of an electrolyte layer of solid electrolytic capacitors which employ a valve metal as an anode.
2. Description of the prior art:
Solid electrolytic capacitors ordinarily possess the structure shown in FIG. 4. This solid electrolytic capacitor 40 is composed of an electrode 4 provided with an anode lead 6; a dielectric oxide film 41, a solid electrolyte layer 42, a carbon layer 43, and a cathode layer 44, which are successively formed on the surface of the electrode 4; a positive terminal 61 connected to the anode lead 6; and a negative terminal 45 connected to the cathode layer 44 by means of an electrically conductive adhesive 46.
In the production of the capacitor, the electrolyte layer 42 is formed on the surface of the oxide film 41 that had been formed by anodic oxidation of the surface of the electrode 4. In more detail, the electrolyte layer 42 is ordinarily formed by one of the following methods.
In the first method, a porous electrode made of a valve metal on which an oxide film 41 had been formed is initially impregnated with a dilute manganese nitrate solution, followed by the pyrolysis at an appropriate temperature, and repeating this procedure several times. By this operation, the manganese nitrate is oxidized into manganese dioxide, which fills the voids of the porous electrode. Next, this electrode is impregnated with a concentrated manganese nitrate solution of high specific gravity, and pyrolysis is performed at an appropriate temperature. This procedure is also repeated several times, thereby forming an electrolyte layer of manganese dioxide on the surface of the electrode.
In the second method, first the voids of a porous electrode with oxide film are filled with manganese dioxide by impregnation of a dilute manganese nitrate solution, as in the first process described above. Then, the electrode is immersed in a slurry prepared by dispersing manganese dioxide powder in a manganese nitrate solution, lifted out from the slurry, heated at an appropriate temperature. This procedure is repeated several times, resulting in the formation of a manganese dioxide electrolyte layer on the surface of the electrode.
In the third method, first, the voids of a porous electrode are filled with manganese dioxide in the same manner as described above. Then this electrode is immersed in a manganese nitrate solution of appropriate concentration, and after lifting the electrode out of the solution, sprinkling manganese dioxide powder onto the surface of the electrode. Then, this electrode is heated to pyrolyze the manganese nitrate. As a result, the manganese dioxide powder adheres to the electrode by means of the manganese dioxide produced by the pyrolysis, thereby forming the desired electrolyte layer of manganese dioxide.
However, these conventional methods involve the following problems. Concerning the first method, the process of immersing the electrode in a manganese nitrate solution followed by pyrolysis of the manganese nitrate must be repeated many times in order to obtain a manganese dioxide electrolyte layer of the desired thickness, which entails high production costs. The formation of a manganese dioxide electrolyte layer with the desired thickness by fewer repetitions of the said process would require a concentrated manganese nitrate solution of higher specific gravity. However, such highly concentrated solutions of manganese nitrate are extremely hygroscopic, which entails considerable difficulties in adjustment of concentration and storage.
As regards the second method, this method has the advantage of decreasing the number of repetitions of the manganese nitrate pyrolysis routine. However, the manganese dioxide in the slurry is difficult to maintain in a uniform state of dispersion over a long period of time, moreover, the adjustment of the constituents of the slurry and storage of the slurry are also difficult. If a uniform slurry is not obtained, the uniform distribution of the manganese dioxide powder over the electrode surface is difficult, and therefore the electrolyte layer so obtained tends to be nonuniform.
With regard to the third method, the application of the manganese dioxide powder at a uniform thickness by sprinkling is difficult, and in particular, portions of the electrode without adhering powder may occur in the shadow of the powder jet from the sprinkling nozzle.